This invention relates to ultrasonic diagnostic systems which measure the velocity of blood flow using spectral Doppler techniques. In particular, the invention relates to the continuous display of such information, including maximum and mean blood flow velocities.
Ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. For blood flow measurements, returning ultrasonic waves are compared to a frequency reference to determine the frequency shifts imparted to the returning waves by moving objects including the vessel walls and the red blood cells inside the vessel. These frequency shifts translate into velocities of motion.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave Doppler waveform is computed and displayed in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
In the conventional spectral Doppler mode, an ultrasound transducer array is activated to transmit by a transmit ultrasound burst which is fired repeatedly at a pulse repetition frequency (PRF). The PRF is typically in the kilohertz range. The return radiofrequency (RF) signals are detected by the transducer elements and then formed into a receive beam by a beamformer. For a digital system, the summed RF signal from each firing is demodulated by a demodulator into its in-phase and quadrature (I/Q) components. The I/Q components are integrated (summed) over a specific time interval and then sampled. The summing interval and transmit burst length together define the length of the sample volume as specified by the user. This so-called xe2x80x9csum and dumpxe2x80x9d operation effectively yields the Doppler signal backscattered from the sample volume. The Doppler signal is passed through a wall filter, which is a high pass filter that rejects any clutter in the signal corresponding to stationary or very slow-moving tissue, including a portion of the vessel wall(s) that might be lying within the sample volume. The filtered output is then fed into a spectrum analyzer, which typically takes the complex Fast Fourier Transform (FFT) over a moving time window of 64 to 256 samples. The data samples within an FFT analysis time window will be referred to hereinafter as an FFT packet. The FFT output contains all the information needed to create the video spectral display as well as the audio output (typical diagnostic Doppler ultrasound frequencies are in the audible range).
For video display, the power spectrum is computed by taking the power, or absolute value squared, of the FFT output. The power spectrum is compressed and then displayed via a gray-scale mapping on the monitor as a single spectral line at a particular time point in the Doppler velocity (frequency) versus time spectrogram. The positive frequency [0:PRF/2] spectrum represents flow velocities towards the transducer, whereas the negative frequency [xe2x88x92PRF/2:0] spectrum represents flow away from the transducer. An automatic Doppler maximum/mean waveform tracing is usually performed after the FFT power spectrum has been compressed. The computed maximum/mean velocity traces are usually presented as overlay information on the spectrogram display.
For the audio Doppler output, the positive and negative frequency portions, or sidebands, of the FFT output are split into two separate channels representing the forward and reverse flow spectra respectively. For each channel, the sideband is reflected about the zero frequency axis to obtain a symmetric spectrum, which generates, after an inverse FFT (IFFT) operation, a real-valued flow signal in the time domain. Both the forward and reverse flow signals are converted into analog waveforms, which are fed to corresponding audio speakers.
During a spectral Doppler exam, the sonographer often needs to move the probe over an anatomical region surrounding some vascular system. Probe motion effects may also result simply from large tissue movements due to breathing or other body motion. Whenever the Doppler sample volume is being jerked around over body tissue, low-frequency clutter is generated which can be significantly stronger than vessel wall signals. Such probe-motion-induced clutter often exceeds the wall filter cutoff frequency, and will show up as blooming white (very strong) echoes right above the wall filter region in the spectral display. This can be considered as the audio counterpart of the xe2x80x9cflash artifactxe2x80x9d in color flow imaging. The annoying effect lies not so much in the video display, but in the Doppler audio: such clutter usually generates a loud rattle that may scare the patient and/or increase his/her anxiety level.
Some form of automatic gain control is usually available in conventional Doppler scanners. Automatic gain control is used to prevent large signals from saturating various points in the signal chain including the video display and/or audio speaker output. Automatic gain control generally consists of detecting the signal amplitude level and adjusting the gain down if the signal approaches a maximum allowable level. Such gain control does not specifically attempt to detect the presence of and to completely mute out the loud probe-motion-induced clutter noise.
There is a need for a Doppler processor that can monitor the I/Q data for the presence of probemotion-induced clutter before the I/Q data is wall filtered.
The present invention is a method and an apparatus for monitoring the wall signal input to the wall filter of a spectral Doppler processor to check for probe-motion-induced clutter. This clutter is typically of higher frequency and amplitude than that due to normal vessel wall motion. In accordance with the preferred embodiment of the invention, some additional threshold logic is used to check for energy within a frequency band greater than the normal wall signal frequencies. If significant energy above some xe2x80x9crattlexe2x80x9d threshold is detected for a predefined time interval, the Doppler audio is automatically muted. This can be effected at one or more points within the normal Doppler audio signal path in a conventional scanner. If the rattling clutter is no longer detected, the Doppler audio is re-activated or ramped up smoothly.
In accordance with the preferred embodiment, a system noise model is used to predict the mean system noise power in a bandpass filter output. The mean system noise power predicted by the system noise model provides a noise threshold to gage how much probe-motion-induced clutter power is present in the current FFT packet. If no significant probe-motion-induced clutter is present, then the audio processing will not be turned off. If significant probe-motion-induced clutter power is present in the FFT packet, the audio processing is turned off.
It should be clear to those skilled in the art that the method of the invention can be implemented in hardware (e.g., a digital signal processing chip) and/or software.